The subject invention relates to control systems and algorithms therefor for the art of gamut mapping and, more particularly, to color registration systems for insuring that colors are printed or displayed with the correct color.
Color correction and/or control should not be confused with color registration systems and sensors for insuring that colors are positioned properly, printed accurately, superposed correctly and/or adjacent to one another.
Electronic color printing or display systems have a continuing need for more accurate outputs not only for achieving consistency in appearance but also assuring that when a special output color needs to be presented, that that output color is precisely accurate. Essentially all print or display color outputs are device dependent, i.e., the unique operating characteristics of that device dictate the precise output color. Accordingly, color generation in electronic systems has been particularly troublesome due to constantly varying factors contributing to the unique outputs, e.g., electronic display or print, lighting, other ambient conditions and particularly for print output devices, tendencies to drift over time. For print media, accurate color generation usually requires test patches and manual calibration wherein a lookup table (LUT) is employed to convert input signals from a scanner, processor or the like to output signals that are used to generate the print output. The generation and maintenance of LUTs can be a time consuming task.
The conversion of the input signals to output signals using such a LUT is generally described as a marking engine. Marking engines can use various color mapping techniques varying from LUTs to analytical functions for effecting the desired gamut mapping. The particular form of color mapping or marking engine employed is not important to this subject invention. This invention is applicable to any type of conversion engine.
Although the subject disclosure is primarily directed to color gamut mapping, it should be appreciated by one of ordinary skill in the art, that the transforming techniques disclosed herein can be applied to any input/output system wherein certain output control values must be precisely maintained with respect to certain critical input control values, and output values resulting from inputs near these critical input control values are best warped for a relative smooth dependency of output control values on these nearby input control values, but where the conversion function itself need not be globally adjusted. The subject invention has particular applicability in complex conversion algorithms such as in color mapping where three or four dimensional nonlinear mapping is involved.
With particular attention to digital color printing systems, when a printer has an output image including certain xe2x80x9ccriticalxe2x80x9d colors, the printer must adjust the output so that the critical colors are precisely and accurately generated. A common example would be an art agency generating a promotional image including a color that the advertiser desires to be especially accurate so it is to be rendered with the exact pantone colors, e.g., yellow and red in a Kodak(copyright) film box, Coca-Cola(copyright) red versus Pepsi(copyright) red. Such exact control usually requires adjustment of corresponding input color signals to the print engine by a feedback control loop to insure that the engine output provides the exact match. A resulting problem with this particular limited and selective matching is that print output colors near the critical color continue to be generated pursuant to the normal operation of the marking engine and thus without any adjustment relative to the critical color. More particularly, the mapping of a limited number of critical colors will leave discontinuities in the color spectra near the critical color, thereby causing objectionable image artifacts in the overall print appearance. For example, the shading in the image may not look right due to discontinuities in the color gradients near the critical color. Conventional interpolation routines have not been able to successfully overcome this problem due to their tendencies to affect colors relatively far from the critical colors and generally do not limit the region in color space where colors are changed.
The present invention contemplates a new and improved mapping scheme which overcomes the above problems and others to provide a new hierarchical strategy for locally adjusting colors around a critical color yet allowing larger regions of color space to remain unadjusted, or if there are several critical colors in a region that need to be adjusted in the same direction, the larger region can be appropriately adjusted also. The invention is readily adaptable to a plurality of marking engine schemes and, for that matter, is even applicable to transformation functions beyond color gamut mapping.
In accordance with the present invention, there is provided a mapping system that is particularly useful in a color gamut mapping wherein input colors are mapped to corresponding output colors in accordance with the marking engine. The invention includes a method of locally transforming a set of colors comprising a critical color and nearby colors to the critical color for precisely matching the critical color to a selected output color and warping the nearby colors relative to the critical color for maintaining a smoothness in appearance among the critical colors and the nearby colors. It is assumed that there exists an approximate mapping of input colors to output colors (for example, the identity map) and that it is desired to add a small adjustment to this map. A set of basis functions are selected that locally change the mapping function with the desired smoothness and together can provide all desired adjustments and a cost is selected for each basis function that indicates the undesirability of using that basis function. Some basis functions are selected for affecting a limited portion of color space gamut comprising the critical color and only other nearby colors. Other basis functions are selected for smoothly changing a larger portion of the color space. There may be many possible combinations of basis functions that satisfy the objective of correctly mapping the critical colors and it is the subject of this invention to choose the xe2x80x9cbestxe2x80x9d combination. The balancing between choosing several local basis functions or a few basis functions affecting a larger portion of color space is controlled by the choice of costs associated with each basis function. If high costs are associated with functions affecting a larger portion of color space then these basis functions will be used only if several critical colors can be mapped as a group using these functions.
In accordance with another aspect of the present invention, the basis functions are selected by a user to cause a transforming satisfying predetermined user standards for local adjustment and global smoothness.
In accordance with yet another more limited aspect of the present invention, the choice and weighting of the basis functions are calculated by a solution technique such as minimizing a function of the weights for the basis functions subject to correctly mapping the critical colors.
One benefit obtained from the present invention is a processing technique and a mapping system where certain predetermined critical colors are precisely and accurately output, and adjacent colors to the critical colors are warped relative to the critical colors to provide a smoothness in spectra appearance.
Another benefit obtained from the present invention is a system which provides user ability to select a set of basis functions for the transforming of nearby colors to the critical colors and yet further permits larger regions of color space to be adjusted if there are several critical colors in a region that need to be adjusted in a same direction. By an appropriate choice by the user of a set of basis functions and corresponding weights, complete control of the tradeoffs between local adjustment and global smoothness can be achieved.
Other benefits and advantages for the subject new mapping technique will become apparent to those skilled in the art upon a reading and understanding of the specification.